Data flow overview

The Planck satellite generates (and stores on-board) data continuously at the following typical rates: 21 kilobit s−1 (kbps) of house-keeping (HK) data from all on-board sources, 44 kbps of LFI science data and 72 kbps of HFI science data. The data are brought to ground in a daily pass of approximately 3 h duration. Besides the data downloads, the passes also acquire realtime HK and a 20 min period of real-time science (used to monitor instrument performance during the pass). Planck utilises the two ESA deep-space ground stations in New Norcia (Australia) and Cebreros (Spain), usually the former. Scheduling of the daily telecommunication period is quite stable, with small perturbations due to the need to coordinate the use of the antenna with other ESA satellites (in particular Herschel).

At the ground station the telemetry is received by redundant chains of front-end/back-end equipment. The data flows to the mission operations control centre (MOC) located at ESOC in Darmstadt (Germany), where it is processed by redundant mission control software (MCS) installations and made available to the science ground segment. To reduce bandwidth requirements between the station and ESOC only one set of science telemetry is usually transferred. Software is run post-pass to check the completeness of the data. This software check is also used to build a catalogue of data completeness, which is used by the science ground segment to control its own data transfer process. Where gaps are detected, attempts to fill them are made as an offline activity (normally next working day), the first step being to attempt to reflow the relevant data from station. Early in the mission these gaps were more frequent, with some hundreds of packets affected per week (impact on data return of order 50 ppm) due principally to a combination of software problems with the data ingestion and distribution in the MCS, and imperfect behaviour of the software gap check. Software updates implemented during the mission have improved the situation such that gaps are much rarer, with a total impact on data return well below 1 ppm.

Redump of data from the spacecraft is attempted when there have been losses in the space link. This has only been necessary on three occasions. In each case the spacecraft redump has successfully recovered all the data.

An operational principle of the mission is to avoid impact on the nominal science of a completely missed ground station pass. Commanding continuity is managed by keeping more than 24 h of commanding-timeline queued on-board. The telemetry resides on board the satellite in a ∼60 h circular buffer in solidstate memory, and can be recovered subsequently using the margin in each pass, or more rapidly by seeking additional station coverage after an event. The lost-pass scenario has in fact occurred only once (on 21 December 2009), when snow on the dish at Cebreros led to the loss of the entire pass. A rapid recovery was made by using spare time available on the New Norcia station. Smaller impacts on the pass occur more often (e.g., the first ∼10 min of a pass may be lost due to a station acquisition problem) and these can normally be recovered simply by restarting a software task or rebooting station equipment. Such delays are normally accommodated within the margin of the pass itself, or during the subsequent pass.

All the data downloaded from the satellite, and processed products such as filtered attitude information, are made available each day for retrieval from the MOC by the LFI and HFI Data Processing Centres (DPCs). Typically, the data arrive at the LFI (resp. HFI) DPC 2 (resp. 4) hours after the start of the daily acquisition window. Automated processing of the incoming telemetry is carried out each day by the LFI (resp. HFI) DPCs and yields a daily data quality report which is made available to the rest of the ground segment typically 22 (resp. 14) hours later. More sophisticated processing of the data in each of the two DPCs is described in Zacchei et al. (2011) and Planck HFI Core Team (2011b).

A sketch of the centres involved in the Planck ground segment and the main data exchanges between them.

For more information, see #tauber2010a, #planck2011-1.1.

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References

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1. References

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